NUTS

Nuts are used with anchors, bolts, screws, studs, threaded rods—and on any other fastener that has machine screw threads—to fasten just about anything to anything. There are many different types of nuts that are used in a variety different applications but broadly fall into two categories: plain and locking. What we will call standard nuts are the ones frequently encountered; locking types, in contrast, have a feature that "locks" the nut thread to the mating threads to resist loosening. Common nuts are listed in Table 1, or click on a type above for detailed information.

By definition, "A nut is [a] block or sleeve having an internal thread designed to assemble with the external thread on a bolt, screw, or other threaded part. It may serve as a fastening means, an adjusting means, a means for transmitting motion, or a means for transmitting power with a large mechanical advantage and non-reversible motion."

Unified inch coarse series (UNC, Unified National Coarse) threads tend to be more popular but Unified inch fine (UNF, Unified National Fine) threads are available too. Coarse threaded fasteners have fewer threads per inch than fine threaded fasteners because coarse threads are farther apart. Consequently, the larger the number of threads per inch, the finer the thread. Also, smaller sizes have more threads per inch than larger sizes—see Table 2 for a comparison between UNC and UNF threads per inch by nut size. Unless noted otherwise, right-hand threads are standard, so turn the nut clockwise to install and tighten. Left-hand threads, also called reverse threads, are available and sometimes abbreviated LH for left hand. (Left-hand threads are often used to prevent a rotating part from unscrewing itself…a classic example is bicycle pedals. The left pedal has left-hand threads and the right pedal has right-hand threads. That way, the pedals effectively tighten themselves as you pedal. Turnbuckles also have left- and right-hand threads.)

Size refers to nominal thread diameter of the nut and must match the size of the mating thread. Sizes range from about #0 to 3 1/2"—typically, the smallest nuts are machine screw and the largest are heavy nuts. The size of small inch series fasteners is specified as a number size (often #0, smallest, to #12, largest); larger sizes (usually beginning at 1/4") are specified in inches, usually fractional rather than decimal. Other nut dimensions of interest are width across flats, which is wrench size, and thickness. Occasionally, width across corners is important because that is the largest width dimension of hex and square nuts. Most nut types are not available in the full range of sizes.

Some nuts are chamfered (beveled) on one side only while others are symmetrical and chamfered on both sides (double chamfered).

To ensure full thread engagement with the nut, bolts/screws should be long enough to allow at least two full threads to extend beyond the nut face after tightening. Conversely, there should be two full threads exposed on the head side of the nut to make sure the nut can be properly tightened.

The two most common types of nuts are hex (short for hexagon, which has six sides) and square (four sided) nuts. Hex nuts are more popular but square nuts have their advantages.

Unlike materials such as stainless steel and brass, some steel nuts are available in different strength "grades" as designated by the ASTM (American Society for Testing and Materials) and SAE (Society of Automotive Engineers).

Common materials include steel (unplated and plated—see more about finishes below), stainless steel, brass, silicon bronze, aluminum and nylon. Steel remains the least costly material followed by stainless steel; copper alloys (such as brass and silicon bronze) are the most expensive.

Common finishes for steel are zinc plating and hot dip galvanizing. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. Hot dip galvanized is a thick coating of zinc that protects against corrosion in harsh environments. Stainless steel, though, is a better choice when corrosion is of concern except when submerged in salt water without free oxygen where it can suffer from severe pitting corrosion. Unplated and uncoated steel nuts—referred to as plain finish—may also be available and are susceptible to rust. Not all types are available in all materials and finishes.

When selecting a nut for use with a strength graded bolt, it is very important that the nut be a compatible grade or strength; one grade higher is also acceptable as one source states.

Always follow the manufacturer's recommendations regarding nut reuse, thread lubricants/locking compounds and torque values. Do not apply standard torque values to prevailing torque lock nuts because the joint may not be tightened to the correct clamping force due to the additional friction of the locking element.

It is advisable to match materials and finishes of nuts, washers and bolts/screws. When using hot dipped galvanized bolts, always use hot dipped galvanized nuts, which are tapped oversize (threaded larger than normal) to accommodate the thick zinc coating on the bolts (using a galvanized nut on a non-galvanized bolt will result in an unacceptably loose fit and using a plain finish or zinc plated nut on a hot dip galvanized bolt will result in a fit that is much too tight).

Stainless steel nuts and bolts/screws used together are susceptible to thread galling and seizing. While it may not be completely preventable, it can be substantially reduced. A thread lubricant is the most effective method. Alternatively, stainless steel alloys having different hardnesses—like a 316 nut and a 304 bolt—have less tendency to gall.

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